1. Field of the Invention
The present invention concerns a metallic strip designed for the manufacture of solid electrolyte capacitors and, more particularly, the manufacture of electrolyte capacitors where the anode is made of aluminum. It also concerns the manufacturing method thereof.
2. Description of the Prior Art
Electrolyte capacitors are used above all because of their high capacitance in a restricted volume. At present, there are three classes of electrolyte capacitors, with low CxV product, in the market. There are aluminum capacitors with liquid electrolyte where the anode is made of aluminum and the cathode is an electrolyte liquid. There are also tantalum capacitors with solid electrolyte, where the anode is made of tantalum and the cathode is a solid, semiconducting electrolyte. Finally, there are aluminum capacitors with solid electrolyte: the anode is made of aluminum and the cathode is a solid semiconducting electrolyte.
The last-named class of electrolyte capacitors has undoubtedly seen development recently. The firm Philips has developed several ranges of aluminum electrolyte capacitors with solid electrolyte. The anode is made with a rolled or folded foil. The manufacturing method using a anode in the form of foil comprises the following steps:
cutting out the aluminum foil;
etching the foil;
folding the etched aluminum foil in the case of radial capacitors or rolling it up in the case of axial capacitors,
anodizing the foil to form a thin film of alumina,
forming the solid electrolyte (manganese dioxide) by pyrolysis;
positioning a cathode contact.
The method used to manufacture these capacitors is fairly complicated. It comprises a one-by-one folding operation for radial capacitors or a one-by-one rolling operation for axial capacitors. The step for the formation of the solid electrolyte is the most delicate one. Several cycles of pyrolysis (four in principle) are required, and these cycles must take place under well-determined conditions of temperature and time. It is very difficult to control these operations. Since the base product is a highly aggressive solution of manganese nitrate, the conversion into manganese dioxide has to be very fast. A post-formation operation is needed to repair the layer damaged by the nitrogen dioxide resulting from the pyrolysis.
There is a method, known through French patent FR No. 2 583 216, for the manufacture of electrolyte capacitors comprising a step for the winding of anode and cathode foils and an electrolyte support on a large-diameter wheel. This method has the advantage of simplicity, obtained by the winding method. It can be used to obtain SMC (surface-mounted component) type components. The solid electrolyte used is preferably manganese oxide but the possibility of using an organic electrolyte is also mentioned. For example, it is possible to use 7, 7, 8, 8-tetracyanoquinodimethane (more commonly called TCNQ) salts. These salts are theoretically of great value, but their use as electrolytes for capacitors raises many problems in implementation.
To overcome this drawback, the invention discloses a metallic strip coated by lacquering with an electrolyte layer, thus enabling the manufacture of a new type of solid electrolyte capacitor made by stacking or by rolling.